One advantage of communication over multiple propagation channels is that it is less susceptible to fading than is single-channel communication. Fading is the loss of received signal power due to destructive interference or obstructions in the propagation channel of the signal. The use of multiple propagation channels can mitigate the effects of fading because, if the various channels have statistically independent fading behavior, it will be unlikely for all channels to be equally affected by fading at a given time. Thus, even if some propagation channels are degraded by fading at a given time, it is likely that there will be other channels that have good quality.
Another advantage of the use of multiple propagation channels is that it affords higher capacity. One particular consequence of this is an increase in the practicality of sending redundant information, so that, for example, data corrupted by fading can be corrected.
The availability of alternate propagation channels due to transmission or reception at multiple polarizations is referred to as “polarization diversity.” Polarization diversity is helpful for mitigating fading effects because in scattering environments, mutually orthogonal polarization channels generally suffer fading effects that are at least partially independent. Fading effects are “independent” in this regard if they have a relatively low statistical correlation.
Prior art systems for wireless communication have embodied the long-recognized constraint, imposed by Maxwell's equations, that signals transmitted through free space in a straight line from point A to point B, and differing only in their polarization modes, can comprise at most two independent signal channels. Mathematically stated, the maximum number of independent communication channels cannot exceed the rank of a matrix H whose elements are coefficients relating the electric and magnetic field components at A to the electric and magnetic multipole moments of an electric current distribution localized at B. For free-space, line-of-sight communication between two points, such a matrix has rank 2 in the far-field limit.
As a consequence, a typical antenna of the prior art, designed for multi-channel reception or transmission at a single geographical point, consists of a pair of mutually orthogonal dipole elements, each effective for receiving or transmitting electromagnetic radiation having a corresponding polarization mode. Thus, each dipole element is effective for communicating over a distinct physical propagation channel, characterized by its polarization.
Because it has generally been believed that only two polarization channels are available at a given point, efforts to increase the number of propagation channels have focused on geographically distributed antenna arrays. That is, if a pair of antenna elements are separated by a sufficient distance, typically of about a communication wavelength or more, their respective propagation paths to or from a common receive or transmit antenna, in a scattering environment, will generally suffer fading effects that are at least partially independent. The availability of such alternate channels due to transmission from or reception at multiple, spatially separated antenna elements is referred to as “spatial diversity.”
It has recently been pointed out that in a rich scattering environment, i.e., where a significant fraction of received signal energy comes from scattering paths rather than from direct line-of-sight, there will generally be three, and not two, polarization channels available at any given point. This is explained, for example, in U.S. patent application Ser. No. 09/379,151, filed on Aug. 23, 1999, and in a continuation-in-part thereof having Ser. No. 09/477,972, filed on Jan. 5, 2000, both commonly assigned herewith. As a consequence of the third polarization channel, rich scattering environments potentially offer more polarization diversity than is available for pure line-of-sight communication.
Every increase in diversity has the potential to further improve reception in the presence of fading. For that reason among others, it is advantageous to find still further forms of diversity.